In today's rapidly advancing semiconductor manufacturing industry, the demand for increasing levels of device integration requires that device features are made increasingly smaller and in closer proximity to one another. The most critical step in defining and ultimately producing device features are the photolithography and etching operations. As such, higher levels of device integration will likely be attained by technological advances in the photolithography and/or etch processes. In order to meet this demand, processes for increasing photomask resolution, such as the process of optical proximity correction (OPC), are put forward constantly.
The object of OPC is to eliminate the phenomenon of the proximity effect in photolithography. In metal-oxide-semiconductor (MOS) devices, each of several component layers, i.e., film layers and dopant levels, is patterned using a photolithography step. Photolithography entails coating a substrate, such as a semiconductor wafer, with a photosensitive film, commonly called photoresist, then exposing the photosensitive film by projecting light through a photomask that includes transparent areas and an opaque pattern. The photomask pattern is transferred to the photoresist layer producing a photoresist pattern which acts as a mask for subsequent doping or etching procedures.
A light beam that travels along the edge of an opaque feature produces a scattering phenomenon that enlarges the light beam. When the light beam passes through the photoresist layer on the substrate, it also reflects off the substructure beneath the photoresist layer and the phenomenon of interference results. As such, various phenomenon influence the projection of an opaque pattern from a photomask onto a photoresist layer. The smaller the critical dimension of the pattern features are, the more prominent these phenomenon become, especially when the critical dimension approaches half of the wavelength of the light source.
These exposure phenomenon combine to create the proximity effect which causes problems when isolated lines and nested, i.e., densely packed, lines undergo exposure at the same time. Isolated, or outermost lines in a photomask pattern lack an adjacent opaque scattering feature in the device pattern. The proximity effect causes isolated and nested lines which have the same dimensions in the opaque photomask pattern, to be formed to include different dimensions in the pattern formed in the photoresist material. Isolated (or outermost) lines and nested lines of the same mask dimension therefore require different exposure conditions to produce device features of the same dimension.
The use of scattering bars represents one OPC technique used to correct and reduce the proximity effect in the photolithography process and to correct for mask bias differences between nested and isolated lines. According to this technique, bar-like patterns are added to the photomask along the periphery of, and in close proximity to, actual device features that would not otherwise have an opaque scattering feature in the device pattern. When exposure occurs, the light wave passes the scattering bar and scatters so that the proximity effect is eliminated with respect to outermost or isolated device features. If the scattering bars are not wide enough, they do not sufficiently eliminate the proximity effect or compensate for depth of focus concerns that arise between features at different topographical levels of the device being exposed. Conversely, if the scattering bars are too wide, they undesirably cause a corresponding and undesirable pattern to be formed in the photoresist layer.
It would therefore be desirable to utilize scattering bars sufficiently large to mitigate the proximity effect and depth of focus issues, but which do not remain and cause unwanted pattern features in the layer being patterned.